The present invention relates to a ball screw with inner ball circulation and a linear actuator equipped with the ball screw, and, specifically, to a ball screw formed such that balls can circulate in a ball circulation tube and a ball circulation section and to a linear actuator which linearly moves a housing by the ball screw.
Recently, according to its increased demand and importance in the fields of manufacturing and processing accurate parts, fabricating semiconductors, microsurgery, genetic manipulation, and the like, a study for development and use of a robot capable of accurate and precise work has been performed. Also, development of a linear actuator having a linear driving range with several microns of position accuracy, employed in a robot for performing accurate and precise work, has been studied.
The development of a linear actuator which is equipped with a link connecting a moving platform and a base platform of a parallel manipulator and linearly moves has been developed and used all over the world since 1980s.
As a linear actuator conventionally employed, there are air pressure actuators, oil pressure actuators, piezo-electric actuators, voice coil actuators and the like. These actuators are not used widely since they are difficult to manufacture and their transfer displacement to size ratio is small.
A ball screw according to the conventional art is disclosed in U.S. Pat. No. 5,560,251 entitled xe2x80x9cA BALL NUT AND A SCREW ASSEMBLYxe2x80x9d, and U.S. Pat. No. 4,660,431 entitled xe2x80x9cROTARY MOTION LINEAR ACTUATORxe2x80x9d.
The ball screw according to the conventional art, as shown in FIG. 1, consists of a screw axis 1 formed with a male screw, a plurality of balls 2 contacting the groove of the male screw of the screw axis 1, a housing 3 formed on its cylindrical inner surface with a female thread corresponding to the male screw of the screw axis 1 and surrounding the balls 2, and a outer return path 4.
The outer return path 4 serves as a passage for circulating the balls 2 when the screw axis 1 rotates. The outer return path 4 is formed as a tube surrounding the screw axis 1. The balls 2 continuously roll and circulate between the male screw of the screw axis 1 and the female thread of the housing 3 through the outer return path 4. Such balls 2 serve to minimize the frictional force due to rotation of the screw axis 1.
However, the ball screw according to the conventional art has a problem in that it is not employed in a structure where a density of mechanisms is great, such as a parallel manipulator, since the outer return path formed exterior to the screw axis increases the size of the apparatus.
Also, the linear actuator equipped with the conventional ball screw has a problem in that the links interfere with each other since it is provided between links of the parallel manipulator.
Moreover, the linear actuator equipped with the conventional ball screw is difficult to employ in the fields of small robots, equipments for microsurgery and the like since it is equipped with a relatively large ball screw.
Further more, the linear actuator equipped with the conventional ball screw has a problem with its transfer displacement to size ratio.
Accordingly, the present invention is invented in order to solve the aforementioned problems.
It is an object of the present invention to provide a ball screw with inner ball circulation and a linear actuator equipped with the ball screw in which its transfer displacement to size ratio is improved.
It is another objective of the present invention to provide a ball screw with inner ball circulation and a linear actuator equipped with the ball screw having an optimized size and several microns of position accuracy.
In order to solve the aforementioned problems, the present invention is provided with a ball screw including a cylindrical housing having a cylindrical inner surface formed with a female thread and a plurality of balls arranged in a space between said female thread and a male screw corresponding to said female thread, the ball screw comprising: a ball circulation tube with a male screw formed on its outer cylindrical surface which corresponds to said female thread of said housing and formed with ball openings to penetrate from both ends of said male screw toward an inner part; and a ball circulation section with an inner return path formed on its cylindrical outer surface, whose start point and end point are matched with said ball openings of said ball circulation tube, wherein, when said ball circulation tube and said ball circulation section rotate in unison, said balls circulate along the inner return path of said ball circulation section.
According to the ball screw of the present invention, said ball openings of said ball circulation tube are formed to have a torsion angle of 13xc2x0 with respect to a cross section perpendicular to an axis of said ball circulation tube and a torsion angle of 25xc2x0 with respect to a cross section including the axis of said circulation tube.
According to the ball screw of the present invention, said ball circulation tube is fixed by a coupling nut coupled to said ball circulation section.
Also, according to the present invention, a linear actuator is provided in which a housing of a ball screw, operated by a driving motor fixed with a holder, linearly moves along with a linear guide wherein the size of said ball screw is optimized by said ball screw comprising: a ball circulation tube with a male screw formed on its outer cylindrical surface which corresponds to a female thread of said housing and formed with ball openings to penetrate from both ends of said male screw toward the inner part; and a ball circulation section with an inner return path formed on its cylindrical outer surface whose start point and end point are matched with said ball openings of said ball circulation tube.
According to the linear actuator of the present invention, a closed end surface of said housing is formed with a screw hole into which an equipment bar for said actuator is screw-coupled.
According to the linear actuator of the present invention, said holder is formed with a linear guide of which moving line is equal to that of said linear guide of said housing.